According to Wikipedia, galls (cecidia) are formed by parasitic insects and mites like gall wasps (Neuroterus albipes). At some metamorphic stage, these organisms alter cell division processes in meristematic tissues of their host plants, which creates a tumour on (typically) the surface of leaves, branches or roots. These organisms use the galls as their habitat and/or food source (exploiting the sugars present).

However, I also know that bullhorn acacia and ants form an interspecific mutualistic relationship (with galls), so apparently galls don't need to be parasitic. Are there any other specific plant-insect pairs that have the same type of mutualistic gall relationship as the acacia-ant system?

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    $\begingroup$ Pardon the initial link and thank you for clarifying the question $\endgroup$ – Chimango Chisuwo Dec 22 '15 at 9:30
  • $\begingroup$ Thanks for fixing it. I'll remove the now obsolete comment. $\endgroup$ – fileunderwater Dec 22 '15 at 9:36

Yes, pollinating fig wasps are gall inducing and mutualistic at the same time, and actually essential for the pollination of figs (see e.g Martinson et al., 2015). During the very intricate mutualism, fig wasps deposit eggs in some of the flowers and leave others. The flowers with eggs and later larvae will develop into galls that will produce new wasps but no seeds, while the flowers that lack wasp larvae will produce seeds. However, the hatched females that leave the figs will pick up pollen before leaving, and are essentially the only vector for cross pollinations between fig plants.

(with the aim of expanding the answer later...)

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From what I know of ant-Acacia mutualisms, I've never heard of an ant gall in an Acacia. The link in your question also never mentions a gall. You've likely confused either domatia or Beltian bodies as a gall. Both differ greatly from galls in that domatia or Beltian bodies are created by the plant and not by the insect.

Domatia are small plant-made chambers that house mutualistic insects (though are often invaded by non-mutualists).

Beltian bodies are a type of food body (or plant-made structures containing nutritional substances (often high in protein) that attract a mutualist species of ants in Acacia).

See here or here for more info about galls.

To answer your overarching question of whether there are mutualistic galls, I would have to say no. I do not know of any instances of which this is the case. Further, galls are inherently parasitic (not mutualistic). Even if the swollen cells do not interfere with normal plant function (e.g., as is common in Solidago spp.)1, the parasitic insect (or other arthropod) is still required to chew its way out of the gall. Last I checked, chewing on your host's tissues is never very mutualistic.

1: Crutsinger, G.M., Habenicht, M.N., Classen, A.T., Schweitzer, J.A. and Sanders, N.J., 2008. Galling by Rhopalomyia solidaginis alters Solidago altissima architecture and litter nutrient dynamics in an old-field ecosystem. Plant and soil, 303(1-2), pp.95-103.

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